Concrete Calculator: Cubic Yards & Bags
Use this Concrete Calculator to determine the exact volume of ready-mix concrete in cubic yards or pre-blended bags for your project. Whether you are pouring a patio slab, setting deck posts, or digging continuous grade footings, get your material counts right the first time.
How Much Concrete Do I Need for a Slab?
To calculate concrete needed for a slab, multiply length by width by slab thickness in feet to find total cubic feet, then divide by 27. Add a 5% to 10% waste factor to offset ground unevenness, form bulging, and spillage during placement.
Standard concrete density runs approximately 145 to 150 lbs per cubic foot. Deciding between ready-mix vs bagged concrete comes down to scale: jobs over 1 cubic yard (roughly 45 eighty-pound bags) are cheaper, faster, and structurally stronger when delivered by truck at your target compressive strength (typically 3,000 to 4,000 PSI).
Formulas: Slab, Footing & Column Yardage
Concrete is measured and sold in cubic yards. Use these geometric formulas to convert your field measurements from inches and feet directly into cubic yards:
1. Concrete Slab Yardage Formula
2. Concrete Yardage Calculator for Footings
3. Round Post Holes & Sonotube Cylinders
After calculating base volume, apply your waste factor / overage percentage (multiply raw yardage by 1.10 for a 10% safety buffer).
Concrete Bags Calculator (60 lb vs 80 lb)
Bag Yield Reference: An 80 lb bag yields 0.60 cu ft (45 bags per cubic yard). A 60 lb bag yields 0.45 cu ft (60 bags per cubic yard). A 50 lb bag yields 0.375 cu ft (72 bags per cubic yard).
Worked Example: How Many Bags of Concrete for a 10x10 Slab (4" Thick)?
Step 1: Calculate Volume in Cubic Feet. Convert 4 inches thickness to feet (4 ÷ 12 = 0.333 ft). Multiply: 10 ft × 10 ft × 0.333 ft = 33.33 cubic feet.
Step 2: Convert to Cubic Yards. 33.33 cu ft ÷ 27 = 1.23 cubic yards. Adding a 10% waste buffer yields 1.35 cubic yards.
Step 3: Convert to Bags. 33.33 cu ft + 10% buffer = 36.66 cu ft. Divided by 0.60 cu ft (80 lb bag) = 62 bags (80 lb), or divided by 0.45 cu ft (60 lb bag) = 82 bags (60 lb).
Frequently Asked Questions
How do you calculate how much concrete you need?
Multiply length by width by thickness in feet to get total cubic feet, then divide by 27 to convert to cubic yards. Always add a 10% waste factor to cover subgrade imperfections and form deflection.
How many bags of concrete do I need per cubic yard?
One cubic yard of concrete requires either 45 bags of 80 lb mix or 60 bags of 60 lb mix. For smaller post setups using 50 lb bags, you need 72 bags per yard.
How thick should a concrete slab be for a driveway?
Pour standard passenger vehicle driveways 4 inches thick over a well-compacted gravel base using 4,000 PSI mix. Upgrade to 5 or 6 inches if you regularly park heavy trucks, RVs, or equipment trailers.
How much does a cubic yard of concrete cost?
Ready-mix concrete costs between $125 and $175 per cubic yard delivered, depending on mix PSI and local supply. Short orders below 4 to 5 cubic yards usually incur an extra delivery fee of $60 to $110 per short yard.
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